Automatic gain control circuit



Feb. 16, 1960 z. ATLAS AUTOMATIC CMN CONTROL CIRCUIT Filed 001;# 16.1956 n .Sm

INVENTOR. ZE/VAS ATLAS A T TORNEYS .zenas Anas, Teaneck, NJA,

` aprlicationreiate -Raftlclllar to. sainrcontrol. .cir

y tion 'With the 'drawingsof-whiolr;

IQTOMAHC einem Laboratories, Inc., CliftongNA VwareVV s ,toV ele maticgain control).

, For simplicity ofexplanatio ,-.t iinstant-inventionwill signalincreases-rire arnaugati neef earlier Stages-is an 4IE stage `114.whiclagilriayriai 4he, `discussed n'terms of atelev-isionv receiver;however,

theprinciples herein disclosed are applicble-toany chain of; amplifierssubject to varyi` g 'strengthincoming signals,

possibility of overloading, .andg'noise limi-tations.-V

In a television receiver the incomingsignal is'sequen- `tially;V appliedto a chain of ampliiersfto produce an outj put srgrraluof` the deslredlstrengtl1.'f Ihis'fchain-'conrprises .aradio frequency amplifier, canintermediaterequency going AGC signalj which isused tolreducertheoverall .gaingof'thesys'te'rn. ThisAGG s ignalfis applied to'a sub f'sequent amplifier' in order; l without 'increasiilgvtho llQisI k ,berequired.;r For kweak incoming;signals,y eachamplilier Istageisoperatedatf-its maximum gain.` When thestrength Y i 30 the1over`allga'intof'reesta'blish the VdiSilii nl eYel of output lof, theincoming'sig'nal increasesgtlie overallV gain is re'- duced tomaintainan output signal:ofthe--originalinA t .tensity -Thus,when it isnecessaryto reduce theoverall' gain, there. is a. choice asv towhich'stage orstages should have ltsfgain reduced. v v, i

Each amplifier has inherent Within fitsel various conditionsiwhichproduce a disturbance known Yasfnioise7-v Drzinthecase of a televisionreceiver; v sndwt Itis well Y gain of-zthe irs't or RE'st'ag'e with' arsultar-lt` deterioration of 'the lsignal-to-noise ratioVv in the'outputV signal.

" Since/local stationsl producey a inuch'stron'ger incoming -signal'thanvdistant'stations, many television receivers have asffloc'alQdstain'tirswitch: which additiimallyl decreases! the overall gain for strong local`signa-ls.A i

v It is therefore ioneobjectof myinventionfto provideanimprovedgainfcontrol circuit. I i Y ItA is still .another objectoflmyinventionuto provide a ga'incontrol circuit .whichcontinuously.andautomatically i adjusts the yoverall gain foroptiniurrr.conditionsfoffainplificat'ion and noise. i

It is? a further objecty of .my invention-tofproviders,gain

controlfci'rcuit which yeliminat"esftlfrei ne'cessityz ofA aflocalz`distantsvvitch. .f

The attainment-ofYtheSeAObjCtsE-and ther ill fberef:

'alized from' the following specification; italiens conjunc- Fig. l lisa schematic representatirof autonratic''ga-in selectively p, i Y 5 Showny@si yan lFstagef-Tliefamniied Y ih ill tration l,Showing "For weak',Airicor11ing 'siga `er andsubsequen'ttlliiamp 'Rn amplifie-r Hammam@ te?small -v' fmstf be .-1'e"s`s than; f gr; i ",icurve Agare botlfestablcurve" A is predetermi a occurs' b eiorethe opti@ .in vva;correctly;vgltaaeutadieedvfbyrinleaming signals is 'velyrapplieid ta Salas-tad a5the case 0f medium amplifiers in; the charia- Tha strengthinc-.ominosignaismulv-@amplification-of later amplifiers isfaffected--As th` treilgih 0f the incoming Referring Q 10,I provides 1an.Y in -Y k`coming signa t amplifier S12,

Y i 'aalis-iliaiiariA plied to.-V subsequent "amplifiers .CampigliaSeveralfampliv finally, fied Y for. utilization ly, as-lglgcls 164. i .Y

rsi ,(OrfRFlaiIpli- F.perated at maximum gain, and np signalis Iproaged.' by; vAGC Circuit 1&- aWhenythe incoming Signa-lfincreases to amoderate strength, automatic gainfcjlrcuii `1,8 P ,rQduCeS a'iiegaiiv?signal. In' addition, it frnay beoverlfgaddv by the signal applied toitH by' tl1e-iirStr stage; Intlris case,` the AGG signal must be'applied to'thefiirst'stageflz., even atithe risk of producing morenoise in the output signal! The delay iritroduce'cibefore",reducing-@waarin of thew rst amplier gives rise-tothe phrase--Delayed-AGC.

Inl order. todel'ay they ationfof A v tions of vtheiilnals; a disablingcorningisignal, and ythus small-AGC Y l anda Clamping circuit comprisingpotential@ source 2g,

diode 26; establishes atfiuncton Da-suitable potential i that'is` toopositive to-be--o orneby: small signals d eveloped -ly the'autorilati-'ga-in cont-rol circuit i8. lHow ever; asf the AGGv Sign l'becomes rigore-negative, due to' thezhigh strength incoming signal,it-vv-ill overcome the potentiahestablished at pointyvDyanl apply to RFampliier '12 ainegative'goin'g'sggnal vvhichv-is utilized to increaseits gain-to" `cor'pensateafio-1fthe stronger incoming signal.

flhe'efect of'thisoperanonf may-ebeseen from curve A of Fig.'2.` ForSmall increasesgin strength of the input signal, the overallVgain:decrease's slightly; At a. transition point, v'vhenV theVdelaying;eiectof-thedisabllng circuit 1s overcome', thegaindecreasesore A,rapidly with increasing strength ofthe inc.orrriiiasignal--V V Ashas beenstatedfoverall reduced 1n steps; first by reducing the;V gain ofbsequ amplifiers, and then` at apredet'ermfnedrpom lng thegain oi theearlier amplifier;v Qnce-g the..; st mplifieris madeV responsive to theATGC' signalfgain fsareduc'ed (as shown ter edbythe cire bythe slope ofcurveA) at a ate dah determined bvii0isaa0.ii. i .i Waspreviously statedlocal v, tatlonstgenerally vpro;

ist

Y@limsiainiaah. strangervr lasciarla. -laaalfili .f

itwiially .raduced- 'tapplies a small negand to maintain stations. Thesesignals may be so strong as to completely overload all the circuitry. Inorder to eliminate' this possibility, a local-distant switch 28 shown inFig. 1 is utilized. VFor local Stations of highv signal strength, switch28 is in Yits local position and has no effect. When the switchis initsfdistant position, however, a potential obtained from source 30establishes at point G a positive potential which immunizes the ampliersagainst all AGC signals, even those resulting from disturbances, such asstatic.

In order to overcome the above disadvantages and the need foralocal-dist`ant switch, my invention produces a gain control signal whichis' more effective in compensating for variations in the strength of thelincoming signal. This may be seen in Fig. 3.V l f Y In my invention theincoming'signal from antenna 10 traverses RF amplifier 12, is amplifiedthereby, and is applied to control grid 31 of a subsequentamplifier-lli,i

which may be a gain-controlled IF amplifier stage.' The signall iseventually applied to utilization circuit 16,

4which provides a signal to automatic gain circuit 18, as

previously described. When therstrength of the incoming signalincreases, AGC circuit 18 produces atnegative-going AGC signal whichis'app'lied back to control vgrid 31 of tube 114, as previouslyexplained, to reduce the grid bias of the IF stage, and thus its gain.

The downvolting of-grid 31 reduces the current in tube 114, thusupvolting screen grid 35, causing the potential at point F to approachthat of B-\--l. A positivegoing signal is thus made available.Alternately, this positive-going signal may be obtained from the anodeof tube 114. In either case, this increased potentialis applied overconnection 21 to a voltage divider 36 whose lower end is suitablybiased. A portion of the increased voltage from voltage divider 36 isapplied to control grid 38 of an amplifier and phase inverter 40. Thenegativegoing correcting signal from tube 40 is applied to voltagedivider 44.

For Weak incoming signals, changes in the gain of RF amplifier have beenprevented by clamp tube 126 in the same manner as previously described.The transition point where RF amplifier 12 has its gain reduced is nowVdetermined by clamp tube 126, the strength of the AGC signal, theoperation of tubes 114 and 40, and the action of voltage divider 44. Assoon as operation passes the transition point, two varying voltagesaffect the gain of RF stage 12. The first is the negative going AGCsignal from AGC source 18, which is similar to the action of the priorart circuit of Fig. l. The second is the negative going correctingsignal obtained from voltage divider 44, this being entirely differentfrom the fixed potential produced by potential source 22 of Fig. 1.

It may be seen from Fig. 3 that during quiescence, isolating resistances42 and 43 form a voltage divider between point G, which is relativelypositive, and point H, which is relatively negative. Thus, point D has apotential which is intermediate G and H. This corresponds to the priorart quiescent condition. The situation as produced by my invention forincoming signals maintains the same relatively negative potential atpoint G, but lowers the potential at point H in accordance with thestrength of the incoming signal. Since the -potential at point H dropsrapidly with increasing strength of the incoming signal, due to theoperation at screen grid 35 of tube 114, and at tube 40,the resultantpotential at point D drops much more rapidly than in the prior artcircuitry. f Y l A Referring again to Fig. 2, dashed curve B shows theoperation Aas improved by my invention. The portion of the curve beyondthe transition point is much steeper, thus allowing the transition pointtoA be moved to the right. This movement permits a longer delay beforethe RF amplifier becomes responsive to the AGC signal, thus permittingthe positionrof the transition point to'be established bynoise'considerations. Since the slope of curve B is nowadjustableindependently of amplification requirement of RF amplifier 12,any desired combination of transition point position and slope may beobtained for optimum operation from the point of view of noise and ofoverloading.

Thus, the improved circuitprovided by my invention applies to RF stage12, a more Yeffective delayed AGC signal due to the fact vthat apreviously fixed potential has been replaced by a potential which varieswith the strength'ofthe'incoming signal. v

It will be recalled that in Fig. l, local-distant switch 28 provided apositivepotential for wcalc incoming signais. My` invention has -fanadditional vfeature vinv that itV eliminates the. svvitch, and providesa second varying strength negative'goiug correcting signal for stronginput signals. This may be seen in Fig. 3 where voltage divider 44provides for strong incoming signais, a negative going. signal Lwhichfisapplied over connection 46 to point G, there kadding to` the AGC signalapplied't control grid 31`of tube 114.

The foregoing specification has disclosed the principles of my inventionand oneembodiment thereof. Since modifications will loccur to. thoseskilled in the art', `I desire to be limitednot by the foregoingillustrations, but rather by the claims granted to me.

What is claimed is: l y' l. A gaincontrol circuit for a chain ofamplifiers coniprising: a first amplifier of said chain having an inputcircuit and an output circuit; a second amplifier'of said chain having`au'input circuit and an output circuit, said output circuit of saidsecond amplifier comprising a screen grid and a first voltage dividerlconnected to said screen grid to be energized thereby; a connectionbetween the output circuit of said first Aamplifier and the inputcircuit of said second amplifier; an electron discharge device having aninput circuitand an output circuit, said output circuit of said devicecomprising a second voltage divider.; a connection betweenV the outputterminal of said first voltage divider and said input circuit of saiddevice; and a connection between the output terminal of said secondvoltage dividerand said input circuit of said first amplifier.

V2. .The circuit of claim-1 including a connection from said secondvoltage divider to the inputcircuit of said second amplifier.

3. A delayed gain control circuit for a chain of amf plifiers adapted toamplify au incoming signal, comprising: a first amplifier having aninput and an output circuit; a second amplifier comprising a pentode,and having an in put and an output circuit, said output circuitcomprising a first voltage divider connected between the screen of saidpeutode and a source of bias voltage; a connection between said outputcircuit of said first amplifier and the input circuit of said secondamplifier; means energized by one amplifier of said chain to provide again control signal; a connection between the output of said gaincontrol producing means and said input circuit of said second amplifierto control the gain thereof; an electron discharge device having aninput circuit and 'an output circuit, said output circuit comprising asecond voltage divider; a connection between the output terminal of saidfirst voltage divider and said inputcircuit of saiddevice; a connectionbetween the output terminal of said second voltage divider and saidinput circuit of said first amplifier; and delay means to temporarilyimmunize said first amplifier to the output of said device.

4. The, circuit of `claim 3 including a connection be.- tween saidsecond voltage divider -and said input circuit of said second amplifier.v'

5. A delayed gain control circuit for a chain Vof? am,- plifiers,comprising: a first'amplifier of said chain having input and outputcircuits; a second amplifier of said chain having input and outputcircuits; a connection between said output circuit of said firstamplifier and said input circuit of said second amplifier; ,meansenergized by an i amplifier of said chain for producing a gain controlsignal;

means to apply said gain control signal to said input circuit of saidsecond ampliier, whereby strong incoming signals reduce the output ofsaid second aniplienmeans to obtain a correcting signal from said secondamplifier in response to said gain control signal; `means to arri plifysaid correcting signal; and means to apply said amplified correctingsignal to said inpwt circuit of said first amplier. Y Y

References Cited in the le of this patent NITED STATES PATENTS Koch Jan.17, 1939 r Artzt Apr. 30, 1940 Belar Dec. 8, 1942 Bass July 3, 1951Cotsworth Ian. 18, 1955 Schade Aug. 21, 1956 Stenning Mar. 26, 1957`UNITED STATES PATENT OFFICE l CERTIFICATE 0F CORRECTION Patent No,2,925,476 February 16g, 1960 Zenas Atlas It is hereby certified thaterror appears in the printed specification of the above numbered patent'requiring correction and that the said Letters Patent should -readascorrected below.

Column 2, line 48, yfor "increase"` read decrease Signed and sealed this9th day of August 1960o (SEAL) Attest:

KARL H.Y AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

